Shi’ite Muslims marked Ashura on Thursday, a day of pain, pilgrimage, and pageantry that is one of the holiest in their religion.

The word Ashura means 10, and the holiday is the tenth day of Muharram, the first month of the Islamic calendar. For Shi’ite Muslims, those ten days are a period of mourning and remembrance, where they commemorate the death of Hussein ibn Ali, the grandson of the Prophet Muhammad. One of the central figures in the Shi’ite denomination, Hussein was beheaded in the Battle of Karbala in the year 680, and his martyrdom was a defining event in the split between Sunnis and Shi’ites.

Like all religious holidays, the rituals and ceremonies marking Ashura vary greatly. In some countries, men flagellate themselves; in acts of solidarity with Hussein and his family, they beat their backs with blades dangling from chains and slice their scalps with swords to allow their blood to run freely. Some Shi’ite leaders have condemned the practice, and instead organize blood donation drives as an alternative.

In the days leading up to Ashura, thousands of Shi’ites make the pilgrimage to Imam Hussein Shrine in Karbala, Iraq. During the first week of Muharram, they pile into buses and vans and walk by the thousands to visit the holy mosque. In the winter of 2006, when I led a platoon of American soldiers patrolling south of Baghdad, one mission was to guard the pilgrims on their journey. They walked south from Baghdad, thousands each day, chanting and marching as they made their way to the holy city. Groups of men beat themselves with blades and their blood spilled onto the pavement of Highway 1. I didn’t think they could continue, but they barely slowed down, driven by a collective pain and a desire to memorialize a great loss.

Shi’ites believe that Hussein was the rightful successor as the leader of Islam, and his death is a tragedy that that is fixed in their collective memory. According to Vali Nasr, a scholar of the Middle East, Shi’ites remember Hussein as “an innocent spiritual figure…in many ways martyred by a far more powerful, unjust force. He becomes the crystallizing force around which a faith takes form and takes inspiration.”